1. Field of Invention
The invention relates to methods and apparatus for controlling the resonant frequency of optical interferometer cavities and more particularly to laser frequency control.
2. Description of Prior Art
In the field of optics there are many instances in which the scientist wants to effect precisely controlled changes in the physical dimensions of optical interferometer cavities. In this field, control of distances down to 10.sup.-5 microns can have significance. In the past, four types of electro-mechanical transducers were used to produce these dimensional changes and thus alter the resonant frequency of optical interferometer cavities. The four categories of prior art electro-mechanical transducers are discussed below:
A. Electromagnetic. This type of transducer generates a force through the interaction of a current-carrying coil and a magnetic field. The forces produced by these transducers are generally small compared to the other categories, and while these small forces can produce considerable extension in a high compliance system, they are generally insufficient to meet the requirements for controlling a rigid structure such as a laser tube. For this reason electromagnetic transducers seem to have found little application in interferometry or laser frequency control systems.
B. Magnetostrictive. These transducers use the current in a magnetizing coil to expand or shrink a solid magnetic material. Invar is generally used when expansion is desired and nickel when shrinkage is desired. This type of transducer has been used to control the mirrors in a laser cavity but has a relatively limited range (at best 60 micro inches per inch or 0.6 microns per centimeter) and has problems associated with magnetic hysterisis. This latter property, whereby it is possible to have one of two different positions (depending on prior history) for a given magnetizing current, makes the application of magnetostriction very impractical in any kind of servo-control system.
C. Electrothermal. These transducers use electrical current to develop heat in a resistance element which, in turn, brings about thermal expansion of an attached structure. Examples of this technique as applied to laser cavities are the coils, either helical or bifilar helical (no magnetic field), wound around the laser tube itself to control the length of the tube and hence the output frequency of the laser. This type of transducer is capable of producing relatively large extensions, depending, of course, on the permissible temperature range and the expansion coefficient of the structure to which it is applied.
The use of electrothermal transducers is by far the least expensive means of producing controlled changes in the dimensions of optical interferometer cavities and has the added advantage that it can be easily tailored to operate with all common voltage/current supplies. It has the disadvantage, however, of not only requiring power, but being relatively slow in its response time. Before any heat can begin to be transferred to the requisite attached structure, the heater itself must begin to warm up. In response to a step increase in heater power, the extension of the structure will thus initially increase only as a quadratic function of time, at best. Indeed where a helically wound heater coil is firmly bonded to the substrate tube structure the initial heating and expansion of the coil will bring about an expansion of the substrate tube diameter and, through the inverse Poisson effect, a reduction in its length, until a transfer of heat to the substrate reverses this trend and increases the tube length. Even after the heater power is returned by a step decrease to normal, heat will continue to flow into the structure and extend it, until the temperature of the heater is no longer above that of the substrate structure. This type of electrothermal transducer is thus difficult to use in any servosystem because of overshoot or oscillation. Settling times are generally of the order of seconds at best.
D. Piezoelectric. These transducers utilize special materials which contract in length when an electric field is applied. They consume no power and therefore produce no heat and these transducers are capable of extremely fast response. Piezoelectric transducers have found wide application in the optical industry. Their principal drawbacks are high cost and limited sensitivity (typically 5 .times. 10.sup.-3 microns per volt, 0.2 micro-inches per volt), so that the voltages up to 1 kilovolt are often used for appreciable motion. Because they produce a displacement that is a linear function of the voltage applied to them with a frequency response that extends into the low kHz region, they have found a wide application in servo-control systems.